I have sleep paralysis pretty often and I see some sort of demon that I guess tries to scare my body from its paralyzed state. I'm curious as to what people thought about it before.
The prevailing theory is that stories of demonic possession and attack arose at least in part from the experiences of people suffering from sleep paralysis. That is, ancient peoples took the experience at face value. The word nightmares comes from the monstrous horses that would lay on people's chests at night preventing them from moving. I'm drawing this from Carl Sagan's Demon Haunted World which doesn't go into great specifics.